This invention relates to the use of partially crystalline, randomly copolymerized polypropylene for the production of electric cable insulation.
Homopolymeric crystalline polypropylene is unsuitable for cable jacketing due to its hardness, rigidity, and brittleness, especially at low temperature. Therefore, cable jacket insulation has heretofore been manufactured from crystalline, polymeric compositions of sequence-type, i.e. block, polymers made up of propylene and/or propylene copolymers containing between 10 and 30% by weight of polymerized ethylene and having a melt flow index (230.degree. C./2.16 kg. load) of between 0.01 and 0.5 (British Pat. No. 1,328,102).
These polymeric compositions of sequence-type polymers having a high ethylene content are, however, difficult to manufacture on the one hand, and difficult to process on the other hand. In particular, they are not amenable to operations for insulating electrical conductors at high operating speed, e.g., 10 to 100 meters per minute, for example telephone cable, field cable, etc. Moreover, these sequence-type polymers, despite their high ethylene content, are still undesirably hard and stiff for cable jacketing. Furthermore, the sequence-type polymers exhibit very low elongation values which are relatively unsatisfactory for cable jacketing, and these elongation values are even further lowered by aging. Still further, sequence-type polymers, being multiphase systems, exhibit differing shrinkage behaviors depending on their composition.
Consequently, there is a need for a readily processable material for the production of cable jacket insulation which is not too stiff and which has a low shrinkage and high elongation values even after aging.